Charged aerosol detection is a popular and valuable technique for the detection and quantification of substances present in a liquid sample stream, and is particularly well-suited to use in connection with liquid chromatography applications. Briefly described, a charged aerosol detection (CAD) system consists of a nebulizer for generating a spray of droplets from a liquid sample stream (for example, the effluent from a chromatographic column), a discharge source for selectively charging the nonvolatile residue particles produced by drying the droplet spray, and a collector, where the aggregate charge imparted to the particles is measured using an electrometer. The resultant signal is representative of the concentration of the nonvolatile components of the sample stream. CAD is sometimes referred to as a “universal” detection technique, as it is capable of quantifying a wide variety of nonvolatile substances with consistent response. Further details regarding the design, operation and advantages of CAD systems are set forth in U.S. Pat. No. 6,568,245 by Kaufman (“Evaporative Electrical Detector”), the disclosure of which is incorporated herein by reference.
The performance of a CAD system is closely tied to the parameters of the droplet spray produced within the nebulizer. It is generally desirable to generate a spray of droplets of uniformly small diameters, since droplets having relatively large diameters may not have adequate time to dry (i.e., to fully evaporate the solvent) and form particles of non-volatile residue prior to reaching the discharge source, which may in turn compromise the ability of the CAD system to quantify analytes contained within the droplets with high degrees of sensitivity and reproducibility. Prior art CAD systems, such as the one described in the aforementioned Kaufman patent, have commonly utilized a nebulizer in which the liquid sample stream is introduced transversely to a jet of compressed air, which is directed at a velocity sufficient to break the sample stream into droplets. Spray generation is assisted by the action of an impactor, which is located proximate to the intersection of the liquid and gas flows. The smaller droplets travel within the nebulizer under the influence of the entraining gas to the nebulizer exit, and pass thereafter to the discharge source. Larger droplets, having greater momentum, impinge on the surface of the impactor and are either broken into smaller droplets or removed to waste through a drain positioned below the impactor.
While prior art CAD system nebulizers have produced generally acceptable results, there remains a need in the art for improved nebulizer designs in order to facilitate more sensitive and reliable detection.